In semiconductor memory devices such as a NAND flash memory, memory cells are more and more downscaled. When the memory cells are downscaled, a threshold voltage window (hereinafter, also “Vt window”) of the memory cells is narrowed. When the Vt window of the memory cells is narrowed, signal differences (voltage differences) of read data are reduced and thus sense amplifiers may detect data erroneously.
To expand the Vt window of the memory, it is conceivable to use metal floating gates as floating gates. However, the metal floating gates are oxidized in the manufacturing process. Accordingly, the metal floating gates cannot continue to be metals.